San’a, Yemen ? The U.S. and Britain locked up their embassies in Yemen on Sunday after fresh threats from al-Qaida, and the White House expressed alarm at the terror group’s expanded reach in the poor Arab nation where an offshoot apparently ordered the Christmas Day plot against a U.S. airliner.

President Barack Obama’s top counterterrorism adviser, John Brennan, cited “indications al-Qaida is planning to carry out an attack against a target” in the capital, possibly the embassy, and estimated the group had several hundred members in Yemen. Security reasons led Britain to act, too; it was not known when the embassies would reopen.

The U.S. is worried about the spread of terrorism in Yemen, a U.S. ally and aid recipient, Brennan said, but doesn’t consider the country a second front with Afghanistan and Pakistan in the fight against terrorism.

As to whether U.S. troops might be sent to Yemen, Brennan replied: “We’re not talking about that at this point at all.” He pledged to provide the Yemeni government with “the wherewithal” to take down al-Qaida.

Britain and the United States are assisting a counterterrorism police unit in Yemen as fears grow about the increasing threat of international terrorism originating from the country.

The Obama administration claims that the suspect in the plot against the Detroit-bound plane was trained and armed by the al-Qaida affiliate in Yemen. Brennan blamed a series of what he called lapses and human errors in U.S. intelligence and security defenses for allowing a Nigerian man to board the plane with explosives. Passengers and crew subdued the suspect when he tried to set off the explosion as the aircraft approached Detroit on Dec. 25; he succeeded only in starting a fire on himself.

Yemen is a poor, decentralized and predominantly Muslim country on the Arabian Peninsula. It is the ancestral homeland of al-Qaida leader Osama bin Laden, and the site of the 2000 bombing of the USS Cole, which killed 17 U.S. sailors. A 2008 attack on the U.S. Embassy killed one American.